Optical discs having a recording density of about 50 GB are now commercially available even for public use, which discs use blue-violet semiconductor laser beams and rely upon Blu-Ray Disc (BD) specifications, High Definition Digital Versatile Disc (HD DVD) specifications and the like.
Now, optical discs that will have a capacity as large as a Hard Disc Drive (HDD) having a capacity of e.g., 100 GB to 1 TB are desired.
However, in order to realize such an ultra high density of an optical disc, new storage techniques are required which are different from conventional high density techniques which resort to a shorter wavelength and a high objective lens NA.
Researches of next generation storage techniques are under progress, and holographic recording techniques have been paid attention.
The holographic recording techniques are techniques of superposing in a recording medium a signal beam having information on page data two-dimensionally modulated by a spatial light modulator and a reference beam and causing refraction index modulation in the recording medium in accordance with an interference fringe pattern formed upon superposition to thereby record information in the recording medium.
For information reproduction, upon application of the reference beam used for recording to the recording medium in the same layout, a diffraction beam is generated because holograms recorded in the recording medium function as diffraction grating. This diffraction beam is reproduced as the same beam as the recorded signal beam including phase information.
The reproduced signal beam is detected two-dimensionally at high speed with an optical detector such as CMOS and CCD. According to the holographic recording techniques, by using one hologram, two-dimensional information can be recorded in an optical recording medium at a time and this information can be reproduced. Since a plurality of sets of page data can be written in a superposed manner in the same area of a recording medium, it is very effective for recording/reproducing information of a large capacity at high speed.
Hologram recording techniques are described, for example, in JP-A-2004-272268 (Patent Document 1). This publication describes a so-called angle multiplex recording method by which a signal beam is converged to an optical information recording medium via a lens, at the same time a reference beam as a parallel beam is applied to be interfered with the signal beam and record a hologram, and while an incidence angle of the reference beam upon the optical information recording medium is changed, different page data is displayed on a spatial light modulator to perform multiplex recording. This publication also describes the techniques that a space between adjacent holograms can be made short by disposing an aperture or opening (spatial filter) at a beam waist of the signal beam converged by a lens so that recording density/capacity can be increased more than a conventional angle multiplex recording method.
Hologram recording techniques are also described, for example, in WO2004-102542 (Patent Document 2). This publication describes an example utilizing a shift multiplex method. In this method, light from an inner side pixel of one spatial light modulator is used as a signal beam, light from an outer side pixel in a ring stripe is used as a reference beam, both the beams are converged to an optical recording medium via the same lens to make both the beams interfere each other near at a focal point plane of the lens to thereby record a hologram.